How to Remove Entry from Map in Kotlin: Simple Guide

In Kotlin, you can remove an entry from a mutable map using the remove(key) function, which deletes the entry with the specified key. This function returns the removed value or null if the key was not found.

Syntax

The basic syntax to remove an entry from a mutable map is:

• map.remove(key): Removes the entry with the given key from the map.
• Returns the value associated with the removed key, or null if the key was not present.

kotlin

val map = mutableMapOf("a" to 1, "b" to 2)
val removedValue = map.remove("a")

Example

This example shows how to create a mutable map, remove an entry by key, and print the map before and after removal.

kotlin

fun main() {
    val map = mutableMapOf("apple" to 10, "banana" to 20, "cherry" to 30)
    println("Original map: $map")

    val removed = map.remove("banana")
    println("Removed value: $removed")
    println("Map after removal: $map")
}

Output

Original map: {apple=10, banana=20, cherry=30}
Removed value: 20
Map after removal: {apple=10, cherry=30}

Common Pitfalls

One common mistake is trying to remove an entry from an immutable map, which will cause a compilation error because immutable maps do not support modification.

Also, calling remove(key) on a mutable map with a key that does not exist returns null and does not change the map.

kotlin

fun main() {
    val immutableMap = mapOf("x" to 1, "y" to 2)
    // immutableMap.remove("x") // This line would cause a compile error

    val mutableMap = mutableMapOf("x" to 1, "y" to 2)
    val result = mutableMap.remove("z")
    println("Result of removing non-existing key: $result")
    println("Map after trying to remove non-existing key: $mutableMap")
}

Output

Result of removing non-existing key: null
Map after trying to remove non-existing key: {x=1, y=2}

Quick Reference

Operation	Description	Example
Remove entry	Deletes entry by key from mutable map	map.remove("key")
Return value	Returns removed value or null if key not found	val v = map.remove("key")
Immutable map	Cannot remove entries; causes compile error	val map = mapOf(...); map.remove("key") // error

Key Takeaways

Use mutableMapOf and remove(key) to delete entries from a Kotlin map.

remove(key) returns the removed value or null if the key is absent.

Immutable maps cannot be modified; use mutable maps to remove entries.

Removing a non-existing key does not change the map and returns null.